The Staphylococci

Staphylococci are Gram-positive spherical bacteria that occur in microscopic clusters resembling grapes. Bacteriological culture of the nose and skin of normal humans invariably yields Staphylococci. In 1884, Rosenbach described the two pigmented colony types of staphylococci and proposed the appropriate nomenclature: Staphylococcus aureus (yellow) and Staphylococcus albus (white). The latter species is now named Staphylococcus epidermidis. Although more than 20 species of Staphylococcus are described in Bergey's Manual (2001), only Staphylococcus aureus and Staphylococcus epidermidis are significant in their interactions with humans. S. aureus colonizes mainly the nasal passages, but it may be found regularly in most other anatomical locales. S epidermidis is an inhabitant of the skin.

Taxonomically, the genus Staphylococcus is in the Bacterial family Staphylococcaceae, which includes three lesser known genera, Gamella, Macrococcus and Salinicoccus. The best-known of its nearby phylogenetic relatives are the members of the genus Bacillus in the family Bacillaceae, which is on the same level as the family Staphylococcaceae. The Listeriaceae are also a nearby family.

Staphylococcus aureus forms a fairly large yellow colony on rich medium, S. epidermidis has a relatively small white colony. S. aureus is often hemolytic on blood agar; S. epidermidis is non hemolytic. Staphylococci are facultative anaerobes that grow by aerobic respiration or by fermentation that yields principally lactic acid. The bacteria are catalase-positive and oxidase-negative. S. aureus can grow at a temperature range of 15 to 45 degrees and at NaCl concentrations as high as 15 percent. Nearly all strains of S. aureus produce the enzyme coagulase: nearly all strains of S. epidermidis lack this enzyme. S. aureus should always be considered a potential pathogen; most strains of S. epidermidis are nonpathogenic and may even play a protective role in their host as normal flora. Staphylococcus epidermidis may be a pathogen in the hospital environment.

Staphylococci are perfectly spherical cells about 1 micrometer in diameter. They grow in clusters because staphylococci divide in two planes. The configuration of the cocci helps to distinguish staphylococci from streptococci, which are slightly oblong cells that usually grow in chains (because they divide in one plane only). The catalase test is important in distinguishing streptococci (catalase-negative) from staphylococci, which are vigorous catalase-producers. The test is performed by adding 3% hydrogen peroxide to a colony on an agar plate or slant. Catalase-positive cultures produce O2 and bubble at once. The test should not be done on blood agar because blood itself contains catalase.

FIGURE 1. Gram stain of Staphylococcus aureus in pustular exudate

Table 1. Important phenotypic characteristics of Staphylococcus aureus
 

• Gram-positive, cluster-forming coccus.
• Nonmotile, non-sporeforming facultative anaerobe.
• Fermentation of glucose produces mainly lactic acid.
• Ferments mannitol (distinguishes from S. epidermidis).
• Catalase positive.
• Coagulase positive.
• Golden yellow colony on agar.
• Normal flora of humans found on nasal passages, skin and mucous membranes.
• Pathogen of humans, causes a wide range of suppurative infections, as well as food poisoning and toxic shock syndrome.